The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,59

different reasonable atmospheres have obtained many other organic compounds, such as sugars, and even the bases that link up to make DNA and RNA, key molecules of terrestrial life. We’ve already mentioned DNA and its double helix, and in any case it’s very well known nowadays. RNA, which stands for ‘ribonucleic acid’, is less well known: it is like DNA, but simpler. With a few exceptions, RNA forms a single strand instead of two intertwined ones. Specific forms of RNA play vital roles in the development of an organism.

These two molecules could easily have been present in the early seas on our planet; indeed, they were probably unavoidable. In addition, we now know that meteorites contain many of these simple organic compounds; indeed, they can form in empty space. So that’s another sensible source of organic chemicals. In short, small organic molecules were around, in quantity, for reasons that have nothing to do with living organisms.

This simple chemistry, though a promising start, isn’t enough. The key molecules in organisms are far more complicated, involving vastly more atoms arranged in fairly specific ways. Graham Cairns-Smith suggested that clay molecules would be ideal catalysts for turning simple organic compounds into polymers of the kinds found in living things: linking amino acids into peptides and proteins, and possibly linking bases with phosphorus and sugars to form short strings of nucleic acids including RNA and DNA. Again, nothing beyond standard chemistry is required to achieve this, and the processes do not involve living creatures. So it would be surprising if there were not many polymers in the early seas. Getting complex molecules is not a problem. We may have trouble coping with their complexity, but nature just follows the rules; from this, a sort of complexity unavoidably follows.

However, polymers aren’t alive. They don’t reproduce, or even replicate, except in very special situations. (Replication is the making of exact copies; reproduction is the making of inexact copies which nevertheless can themselves reproduce, which is more flexible, but even harder to understand.) Replication or reproduction seem to require not just complexity, but organised complexity, and it’s difficult to see where the organisation can come from. However, some of these special situations can occur entirely naturally with certain clays, which themselves exhibit replication. In a watery environment, little slabs of clay can make stacks of almost identical copies, without any help.

Since the late 1990s many things have changed. At that time, in The Science of Discworld, we paid particular attention to the ideas of Gunther Wächtershäuser. His proposal differed from the by then conventional Miller-Urey primeval soup, which spontaneously produced replicating nucleic acids, the first heredity. Instead, Wächtershäuser proposed that the first thing to happen was metabolism: working biochemistry. He suggested that this might have occurred where there was plenty of sulphur, iron oxide and iron sulphide, plus a suitable source of heat to drive the chemistry. One possible location that possesses these ingredients is an undersea hydrothermal vent, known as a black smoker, where molten rock from the mantle makes its way to the surface through cracks where the ocean floor is spreading. Less dramatically, underwater volcanic vents do the same. Using this kind of iron-oxygen-sulphur chemistry, Wächtershäuser came up with a set of chemical reactions that closely mimicked the Krebs cycle, a central biochemical system in nearly all living things.

In laboratory experiments, his scenario performed reasonably well, though not perfectly. So the theory of the origin of life turned into a kind of primeval pizza, with molecules dotted around on a surface, rather than a primeval soup sloshing around in pools or the open sea. In 1999 we liked this idea because it was different from heredity-first systems: we couldn’t see why they would necessarily replicate – what was in it for them. Moreover, Wächtershäuser was a lawyer as well as a biochemist, and it’s unusual to get good original scientific ideas from a lawyer.

However, since then a different idea, the RNA world, has really taken off. RNA and DNA are both nucleic acids, so named because they are found in the nuclei of cells. There are many other kinds of nucleic acid; some are much simpler than DNA and RNA, and some are much more complicated. Both are long chains formed by joining together four smaller molecular units, called nucleotides. Nucleotides are combinations of bases, which in turn are specific molecules that look like complicated amino acids, linked together by sugars and phosphate. Does that help? We thought not. You can look up all